Pn-junction light-emitting diodes (LEDs) are well known as a type of compound semiconductor light-emitting devices. Examples of such known LEDs include a GaP LED having a substrate, and a gallium phosphide (GaP) light-emitting layer obtained through epitaxial growth of an electrically conductive GaP single crystal atop the substrate; an LED which emits red light or orange-yellow to green light, the LED having a light-emitting layer formed of an aluminum gallium arsenide mixed crystal (AlxGayAs: 0≦X, Y≦1, and X+Y=1), or formed of an aluminum gallium indium phosphide mixed crystal (AlxGayInzP: 0≦X, Y, Z≦1, and X+Y+Z=1); and a LED which emits short-wavelength light (e.g., near-ultraviolet light, blue light, or green light), the LED having a light-emitting layer formed of a Group III nitride semiconductor such as gallium indium nitride (GaαInβN: 0≦α, β≦1, and α+β=1).
For example, the aforementioned AlXGaYInZP LED includes a substrate formed of an electrically conductive p-type or n-type gallium arsenide (GaAs) single crystal, on which an electrically conductive n-type or p-type light-emitting layer is formed. The blue LED includes a substrate formed of a single crystal (e.g., an electrically insulating sapphire (α-Al2O3) single crystal). The short-wavelength LED includes a substrate formed of cubic (3C) or hexagonal (4H or 6H) silicon carbide (SiC).
In general, a dicer or a scriber is employed for preparing individual compound semiconductor light-emitting device chips from a compound semiconductor light-emitting device wafer including such a substrate and numerous compound semiconductor light-emitting devices, the devices being regularly and periodically arranged with separation zones being disposed therebetween. A “dicer” is an apparatus for cutting such a wafer into chips through the following procedure: the wafer is subjected to full-cutting by means of rotation of a disk blade having a diamond tip; or grooves having a width larger than that of the blade tip are formed on the wafer (half-cutting), and then the resultant wafer is subjected to cutting by means of external force. Meanwhile, a “scriber” is an apparatus for cutting such a wafer into chips through the following procedure: very thin lines are scribed on the wafer in, for example, a grid form by use of a needle whose tip is formed of diamond, and then the resultant wafer is subjected to cutting by means of external force. A crystal having a zincblende structure, such as GaP or GaAs, exhibits cleavability along a “110” plane. Therefore, by virtue of such a characteristic feature, a semiconductor wafer formed of, for example, GaAs, GaAlAs, or GaP can be relatively easily separated into chips having a desired shape.
However, a nitride semiconductor, which is stacked on a sapphire substrate or a similar substrate, has a heteroepitaxial structure, and has a large lattice constant mismatch with respect to the sapphire substrate. A sapphire substrate has a hexagonal system, and thus exhibits no cleavability. Sapphire and a nitride semiconductor have a Mohs hardness of about 9; i.e., they are very hard substances. Therefore, a wafer including a sapphire substrate and a nitride semiconductor is difficult to cut into chips by use of a scriber. When such a wafer is subjected to full-cutting by use of a dicer, cracking or chipping tends to occur on the cut surfaces; i.e., the wafer cannot be cut into chips successfully. In some cases, a semiconductor layer formed on the sapphire substrate is exfoliated therefrom.
In order to solve such problems, scribing techniques employing laser irradiation have been proposed. As has been reported, when separation grooves are formed on a compound semiconductor wafer by means of laser irradiation, a light-emitting device is produced at high yield and mass-productivity (see, for example, Japanese Patent No. 3449201, Japanese Patent No. 3230572 and Japanese Patent Application Laid-Open (kokai) No. 11-177139). These techniques produce a light-emitting device having a very good shape. However, in practice, debris attributed to laser processing are deposited on the surface of the thus-produced light-emitting device, resulting in lowered efficiency of extraction of light from the device to the outside. When laser processing is performed on semiconductor layers of a compound semiconductor wafer, debris are deposited on the side surfaces of the semiconductor layers or deposited so as to cover surfaces on which a negative electrode and a positive electrode are to be formed, and therefore, electrical characteristics (e.g., the reverse breakdown voltage) of the resultant light-emitting device are deteriorated.
Therefore, a report has been made to the effect that when a protective film is formed on a laser processing surface and contamination deposited onto the protective film is washed away after the formation of a laser groove to solve the problems described above, Group III nitride type compound semiconductor devices can be acquired with high yield (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-31526). According to this method, electric characteristics such as reverse breakdown voltage can be improved and the drop of the yield resulting from defect of appearance and characteristics can be improved, it is true, but the problem remains unsolved in that molten matters adhere to the side surface of a separation groove when the separation groove is formed by laser processing and the light emission output of the device drops.